Project Summary. Understanding the genetic and environmental regulators of puberty has become a topic of increasing urgency. Recognizing the importance of the pubertal transition for health across the lifespan, NIH/NICHD issued PA-18-033, Characterization of the Adolescent Reproductive Transition to solicit applications for projects that will fill essential gaps in the knowledge base regarding pubertal development, including increasing our understanding of the ?influences of lifestyle factors and environmental exposures.? Our research aims to identify specific and potentially modifiable environmental factors that influence pubertal development; we view genetic and epigenetic data as essential tools for accomplishing that goal. Our project will use data from two independent samples that both combine longitudinal hormonal and epigenetic data with rich measures of environmental risk. The primary discovery sample will be N = 1000 individuals and their parents from the Texas Twin Project. We have already collected considerable baseline data on this cohort, and we will leverage this existing data to build an accelerated longitudinal study spanning adolescence, including a deep phenotyped sub-sample with up to 8 assessment waves. The replication data set is an existing resource of N = 214 children recruited in early puberty and re-assessed after a 2-year interval. Together, these unique datasets will allow us to accomplish three aims. First, we will apply cutting-edge methods in statistical genetics to findings from previous, well-powered genome-wide association studies (GWAS) of reproductive milestones in order to (a) develop and validate more powerful polygenic scores that are genome-wide measures of an individual?s genetic liability toward earlier reproductive development; and (b) probe the extent to which genetic liabilities operate directly through the individual?s own biology versus through the environment provided by parents (?genetic nurture?). Second, we will use longitudinal, co-twin-control data on DNA methylation (DNAm) across the pubertal transition to build a novel epigenetic clock for ?pubertal age? and will validate this clock in an independent replication sample. Developing a molecular biomarker for environmentally-responsive accelerations in pubertal development has the potential to revolutionize puberty research, just as existing epigenetic clocks have revolutionized aging research. Third, we will test how established environmental risk factors for early and accelerated pubertal development (including early life stress, concentrated neighborhood poverty, and biological father absence) interact with genetic influences using a trio of research designs with different strengths and limitations ? twin, measured gene, and epigenetic designs. Overall, the proposed research has potential to have very high impact, by providing the scientific community both with empirical insights regarding the interplay of genetic and environmental regulators of puberty and with openly-available methodological tools (polygenic scoring and epigenetic clock algorithms) that can be broadly applied to study biological and environmental mechanisms of individual variation in pubertal development and its sequelae.